Systemic Acquired Resistance (SAR)
Plants are constantly challenged by a wide variety of pathogenic organisms including viruses, bacteria, fungi, and nematodes. Crop plants are particularly vulnerable because they are usually grown as genetically-uniform monocultures; when disease strikes, losses can be severe. However, most plants have their own innate mechanisms of defense against pathogenic organisms. Natural variation for resistance to plant pathogens has been identified by plant breeders and pathologists and bred into many crop plants. These natural disease resistance genes often provide high levels of resistance to or immunity against pathogens.
Systemic acquired resistance (SAR) is one component of the complex system plants use to defend themselves from pathogens (Hunt and Ryals, Crit. Rev. in Plant Sci. 15, 583-606 (1996), incorporated by reference herein in its entirety; Ryals et al., Plant Cell 8, 1809-1819 (1996), incorporated by reference herein in its entirety). See also, U.S. Pat. No. 5,614,395, incorporated by reference herein in its entirety. SAR is a particularly important aspect of plant-pathogen responses because it is a pathogen-inducible, systemic resistance against a broad spectrum of infectious agents, including viruses, bacteria, and fungi. When the SAR signal transduction pathway is blocked, plants become more susceptible to pathogens that normally cause disease, and they also become susceptible to some infectious agents that would not normally cause disease (Gaffney et al., Science 261, 754-756 (1993), incorporated by reference herein in its entirety; Delaney et al., Science 266, 1247-1250 (1994), incorporated by reference herein in its entirety; Delaney et al., Proc. Natl. Acad. Sci. USA 92, 6602-6606 (1995), incorporated by reference herein in its entirety; Delaney, Plant Phys. 113, 5-12 (1997), incorporated by reference herein in its entirety; Bi et al., Plant J. 8, 235-245 (1995), incorporated by reference herein in its entirety; Mauch-Mani and Slusarenko, Plant Cell 8, 203-212 (1996), incorporated by reference herein in its entirety). These observations indicate that the SAR signal transduction pathway is critical for maintaining plant health.
Conceptually, the SAR response can be divided into two phases. In the initiation phase, a pathogen infection is recognized, and a signal is released that travels through the phloem to distant tissues. This systemic signal is perceived by target cells, which react by expression of both SAR genes and disease resistance. The maintenance phase of SAR refers to the period of time, from weeks up to the entire life of the plant, during which the plant is in a quasi steady state, and disease resistance is maintained (Ryals et al., 1996).
Salicylic acid (SA) accumulation appears to be required for SAR signal transduction. Plants that cannot accumulate SA due to treatment with specific inhibitors, epigenetic repression of phenylalanine ammonia-lyase, or transgenic expression of salicylate hydroxylase, which specifically degrades SA, also cannot induce either SAR gene expression or disease resistance (Gaffney et al., 1993; Delaney et al., 1994; Mauch-Mani and Slusarenko 1996; Maher et al., Proc. Natl. Acad. Sci. USA 91, 7802-7806 (1994), incorporated by reference herein in its entirety; Pallas et al., Plant J. 10, 281-293 (1996), incorporated by reference herein in its entirety). Although it has been suggested that SA might serve as the systemic signal, this is currently controversial and, to date, all that is known for certain is that if SA cannot accumulate, then SAR signal transduction is blocked (Pallas et al., 1996; Shulaev et al., Plant Cell 7, 1691-1701 (1995), incorporated by reference herein in its entirety; Vernooij et al., Plant Cell 6, 959-965 (1994), incorporated by reference herein in its entirety).
Recently, Arabidopsis has emerged as a model system to study SAR (Uknes et al., Plant Cell 4, 645-656 (1992), incorporated by reference herein in its entirety; Uknes et al., Mol. Plant-Microbe Interact. 6, 692-698 (1993), incorporated by reference herein in its entirety; Cameron et al., Plant J. 5, 715-725 (1994), incorporated by reference herein in its entirety; Mauch-Mani and Slusarenko, Mol. Plant-Microbe Interact. 7, 378-383 (1994), incorporated by reference herein in its entirety; Dempsey and Klessig, Bulletin de L'Institut Pasteur 93, 167-186 (1995), incorporated by reference herein in its entirety). It has been demonstrated that SAR can be activated in Arabidopsis by both pathogens and chemicals, such as SA, 2,6-dichloroisonicotinic acid (INA) and benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH) (Uknes et al., 1992; Vernooij et al., Mol. Plant-Microbe Interact. 8, 228-234 (1995), incorporated by reference herein in its entirety; Lawton et al., Plant J. 10, 71-82 (1996), incorporated by reference herein in its entirety). Following treatment with either INA or BTH or pathogen infection, at least three pathogenesis-related (PR) protein genes, namely, PR-1, PR-2, and PR-5 are coordinately induced concomitant with the onset of resistance (Uknes et al., 1992, 1993). In tobacco, the best characterized species, treatment with a pathogen or an immunization compound induces the expression of at least nine sets of genes (Ward et al., Plant Cell 3, 1085-1094 (1991), incorporated by reference herein in its entirety). Transgenic disease-resistant plants have been created by transforming plants with various SAR genes (U.S. Pat. No. 5,614,395).
A number of Arabidopsis mutants have been isolated that have modified SAR signal transduction (Delaney, 1997) The first of these mutants are the so-called lsd (lesions simulating disease) mutants and acd2 (accelerated cell death) (Dietrich et al., Cell 77, 565-577 (1994), incorporated by reference herein in its entirety; Greenberg et al., Cell 77, 551-563 (1994), incorporated by reference herein in its entirety). These mutants all have some degree of spontaneous necrotic lesion formation on their leaves, elevated levels of SA, mRNA accumulation for the SAR genes, and significantly enhanced disease resistance. At least seven different lsd mutants have been isolated and characterized (Dietrich et al., 1994; Weymann et al., Plant Cell 7, 2013-2022 (1995), incorporated by reference herein in its entirety). Another interesting class of mutants are cim (constitutive immunity) mutants (Lawton et al., "The molecular biology of systemic aquired resistance" in Mechanisms of Defence Responses in Plants, B. Fritig and M. Legrand, eds (Dordrecht, The Netherlands: Kluwer Academic Publishers), pp. 422-432 (1993), incorporated by reference herein in its entirety). See also, U.S. patent application Ser. No. 08/648,949 and International PCT Application WO 94/16077, both of which are incorporated by reference herein in their entireties. Like lsd mutants and acd2, cim mutants have elevated SA and SAR gene expression and resistance, but in contrast to lsd or acd2, do not display detectable lesions on their leaves. cpr1 (constitutive expresser of PR genes) may be a type of cim mutant; however, because the presence of microscopic lesions on the leaves of cpr1 has not been ruled out, cpr1 might be a type of lsd mutant (Bowling et al., Plant Cell 6, 1845-1857 (1994), incorporated by reference herein in its entirety).
Mutants have also been isolated that are blocked in SAR signaling. ndr1 (non-race-specific disease resistance) is a mutant that allows growth of both Pseudomonas syringae containing various avirulence genes and also normally avirulent isolates of Peronospora parasitica (Century et al., Proc. Natl. Acad. Sci. USA 92, 6597-6601 (1995), incorporated by reference herein in its entirety). Apparently this mutant is blocked early in SAR signaling. npr1 (nonexpresser of PR genes) is a mutant that cannot induce expression of the SAR signaling pathway following INA treatment (Cao et al., Plant Cell 6, 1583-1592 (1994), incorporated by reference herein in its entirety). eds (enhanced disease susceptibility) mutants have been isolated based on their ability to support bacterial infection following inoculation of a low bacterial concentration (Glazebrook et al., Genetics 143, 973-982 (1996), incorporated by reference herein in its entirety; Parker et al., Plant Cell 8, 2033-2046 (1996), incorporated by reference herein in its entirety). Certain eds mutants are phenotypically very similar to npr1, and, recently, eds5 and eds53 have been shown to be allelic to npr1 (Glazebrook et al., 1996). nim1 (noninducible immunity) is a mutant that supports P. parasitica (i.e., causal agent of downy mildew disease) growth following INA treatment (Delaney et al., 1995; U.S. patent application Ser. No. 08/648,949). Although nim1 can accumulate SA following pathogen infection, it cannot induce SAR gene expression or disease resistance, suggesting that the mutation blocks the pathway downstream of SA. nim1 is also impaired in its ability to respond to INA or BTH, suggesting that the block exists downstream of the action of these chemicals (Delaney et al., 1995; Lawton et al., 1996).
Recently, two allelic Arabidopsis genes have been isolated and characterized, mutants of which are responsible for the nim1 and npr1 phenotypes, respectively (Ryals et al., Plant Cell 9, 425-439 (1997), incorporated by reference herein in its entirety; Cao et al., Cell 88, 57-63 (1997), incorporated by reference herein in its entirety). The wild-type NIM1 gene product is involved in the signal transduction cascade leading to both SAR and gene-for-gene disease resistance in Arabidopsis (Ryals et al., 1997). Ryals et al., 1997 also report the isolation of five additional alleles of nim1 that show a range of phenotypes from weakly impaired in chemically induced PR-1 gene expression and fungal resistance to very strongly blocked. Transformation of the wild-type NPR1 gene into npr1 mutants not only complemented the mutations, restoring the responsiveness of SAR induction with respect to PR-gene expression and disease resistance, but also rendered the transgenic plants more resistant to infection by P. syringae in the absence of SAR induction (Cao et al., 1997).